Brisbane,
There must be something else going on here, as V1.5 is much less processor intensive and much more stable that V1.3.
By the way, unless you uninstalled V1.3, it will still be there on your HD as V.15 installs into a completely separate directory. I just make up a new shortcut to the old directory so that I've got access to different versions. You can run whichever one you like then :-)